A building's windows dramatically affect its energy efficiency. In winter, heat loss through typical windows can boost a building's heating bill as much as 25 percent (see References 1). In summer, the heat flow reverses, with sunlight warming the building's interior through its windows and causing air conditioners to work harder. Energy-efficient windows can help in both these situations. And if the building's electricity comes from a fossil-fuel power plant, installing energy-saving windows also shrinks the building's carbon footprint by reducing greenhouse gas emissions at that plant.
Most normal windows use a single glass pane that readily conducts heat into and out of a building. Normal window frames also cause heat loss if they're made of highly conductive material such as aluminum or if they're not tightly sealed. Energy-efficient windows have multiple panes of glass that minimize heat transfer with insulating air spaces between the panes. Efficiency is increased further when those spaces are filled with nontoxic gases such as krypton or argon and when low-conductivity spacers are used between the panes. (See References 1, 2, 3 and 4)
Applying low-emissivity, or low-e, coatings to glass surfaces is an additional way of controlling heat transfers. When applied to outside glass panes, these microscopically thin metal or metallic oxide layers keep buildings cooler in the summer by reflecting sunlight. In the winter, low-e coatings on interior panes maintain indoor temperatures by preventing heat from escaping. (See References 1 and 5)
Windows and other products with the Energy Star label have met federal guidelines for energy efficiency. The Energy Star program is jointly run by the Environmental Protection Agency and the Department of Energy to promote environmentally friendly energy practices. Use of Energy Star-approved products in 2009 saved an estimated $17 billion on utility bills in the United States and reduced greenhouse gas emissions equivalent to those generated by about 30 million cars. (See References 6)
Building energy-efficient windows into a new home saves money by allowing installation of smaller, less costly cooling and heating equipment. When looking for new windows, check with local utilities for cash rebates and other incentives for installing energy-saving replacement windows. The best windows for cold climates will be different from the most efficient windows for warm or temperate climates. Make sure you select windows constructed for the climate and sunlight conditions where you intend to install them. (See References 1 and 3)
A California-based writer, Mike Williams has written since 1975 on the environment, health care and wildlife for more than 70 media outlets, including the "Detroit Free Press", Agence France-Press and CBS. Williams holds a Bachelor of Arts in journalism from the University of Southern California.